1. Field of the Invention
The present invention relates generally to the manufacture of semiconductor device assemblies. More specifically, the invention pertains to a method and apparatus for measuring the thickness and warpage, or nonplanarity, of substrates for carrying semiconductor devices and the like.
2. State of the Art
In the electronics industry, as well as in many other fields, the suitability of a particular planar member for use may be dependent upon its planarity, i.e., the degree to which a major surface or surfaces are planar. For example, many electronic components are formed as assemblies using semiconductor devices mounted on thin substrates, e.g., circuit boards such as conventional FR-4, FR-5 and BT resin circuit boards. Exemplary semiconductor devices include dynamic random access memory (DRAM) devices, static random access memory (SRAM) devices, flash memory devices, imagers, and many other devices. An exemplary semiconductor device assembly may comprise a memory module comprising a plurality of DRAM semiconductor devices mounted to a substrate in the form of a circuit board.
The thickness of many circuit boards used for such assemblies is generally from about 0.02 inch to about 0.125 inch. The steps in circuit board fabrication generally leave inherent stresses in one or more directions within a circuit board panel from which individual circuit boards are cut. Cutting of a large board panel into smaller discrete circuit boards relieves some of the inherent stress, resulting in some degree of warpage of each circuit board. The attachment of semiconductor devices to a circuit board may also introduce stresses, such as when flip-chip configured semiconductor dice are mechanically and electrically connected to terminals on the circuit board using solder reflow. Although a limited amount of warping may be generally acceptable, as may warpage in particular directions, excessive warpage or warpage in other directions (individually and collectively “unacceptable warpage”) causes several problems. Unacceptable warpage may make installation of a completed assembly into test fixtures or higher-level packaging, such as a motherboard, difficult because of board curvature and nonalignment of electrical connectors. Also, unacceptable warpage affects the assembly packaging process in terms of difficulty in satisfactorily positioning the assembly into an encapsulant mold assembly, and nonuniformity of distribution of encapsulant material on the assembly surfaces and about the base of the semiconductor devices and their connections to circuit board terminals. As a result, completed but defective semiconductor device assemblies may need to be reworked (where possible), discarded or downgraded into products of lower value, reducing overall productivity.
Warpage may occur in any direction. For example, significant warpage may occur simply along an X-axis and/or Y-axis of a substrate. Additionally, warpage may occur in directions noncongruent with the conventional X, Y, and Z axes. Warpage in multiple planes or in directions oblique to planes may be termed “complex” warpage.
Current inspection methods for determining whether a substrate is excessively warped (or otherwise misshaped) generally utilize visual systems. Such systems are characterized by the use of either human vision or machine vision, the latter generally providing a faster and more reliable quantitative measurement of warpage. An example of such a machine vision-based apparatus is shown in U.S. Pat. No. 6,522,777 of Paulsen et al. Such systems are complex, fairly slow, very expensive, and have a degree of accuracy which is generally beyond that required in determining warpage. Use of such machines in determining thickness and warpage of semiconductor device assemblies such as memory modules adds significantly to the cost of production.
There exist various types of equipment for measuring thickness. The common manual caliper may be useful for product development but is too time-consuming for production operations. The web measuring gauges common in the paper industry are useful for determining thickness of sheet materials moving at high speed. Such machines are shown in U.S. Pat. Nos. 4,791,367; 5,132,619 of Typpo, and U.S. Pat. No. 6,145,211 of Typpo et al. None of these machines is designed for, or capable of, measuring warpage in a substantially rigid substrate.
It would thus be desirable to provide an apparatus and method for determining unidirectional or multidirectional warpage in a substrate such as a circuit board.
It would also be desirable to provide an apparatus and method for simultaneous measurement of the warpage and thickness of a substrate.
It would be further desirable to provide an automated apparatus and method for determining the warpage of each of a stream of substrates passing through an assembly line or other continuous manufacturing environment and which, if desired, may be incorporated into existing equipment.
In addition, it would be desirable to provide an apparatus and method for determining linear and/or complex warpage of a stream of substrates in a single measurement pass for each substrate.